EP1365302B1 - Low drop-out voltage regulator - Google Patents
Low drop-out voltage regulator Download PDFInfo
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- EP1365302B1 EP1365302B1 EP03101432A EP03101432A EP1365302B1 EP 1365302 B1 EP1365302 B1 EP 1365302B1 EP 03101432 A EP03101432 A EP 03101432A EP 03101432 A EP03101432 A EP 03101432A EP 1365302 B1 EP1365302 B1 EP 1365302B1
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- 239000002131 composite material Substances 0.000 claims description 19
- 239000003990 capacitor Substances 0.000 claims description 13
- 230000008878 coupling Effects 0.000 claims description 3
- 238000010168 coupling process Methods 0.000 claims description 3
- 238000005859 coupling reaction Methods 0.000 claims description 3
- 239000000872 buffer Substances 0.000 description 10
- 230000001052 transient effect Effects 0.000 description 9
- 238000010586 diagram Methods 0.000 description 8
- 230000007423 decrease Effects 0.000 description 7
- 230000003071 parasitic effect Effects 0.000 description 7
- 230000001105 regulatory effect Effects 0.000 description 3
- 230000004044 response Effects 0.000 description 3
- 230000001276 controlling effect Effects 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 2
- 230000007704 transition Effects 0.000 description 2
- 230000033228 biological regulation Effects 0.000 description 1
- 230000001413 cellular effect Effects 0.000 description 1
- 230000003116 impacting effect Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
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- 230000004048 modification Effects 0.000 description 1
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- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
- G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
- G05F1/10—Regulating voltage or current
- G05F1/46—Regulating voltage or current wherein the variable actually regulated by the final control device is dc
- G05F1/56—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using semiconductor devices in series with the load as final control devices
- G05F1/575—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using semiconductor devices in series with the load as final control devices characterised by the feedback circuit
Definitions
- pass device 104 includes large parasitic capacitances, thus often requiring the implementation of a buffer, or a g m boost, to isolate the high output resistance of the gain stage of error amplifier 102 from the high load capacitance of pass device 104.
- a low drop-out regulator 200 implementing a buffer 206 between the output of an error amplifier 202 and a pass device 204 is illustrated.
- Buffer 206 is configured to receive the output current from error amplifier 202 and drive the gate of pass device 204.
- the output from buffer 206 can be mirrored back through a complex, current mirror circuit comprising transistors M 1 through M 5 to compensate error amplifier 202.
- Input transistor devices 602 and 604 are configured for receiving input current signals, such as from voltage terminals V PP (+) and V NN (-), at their sources, respectively, with the source of input transistor device 602 comprising the positive or non-inverting input terminal and the source of input transistor device 604 comprising the negative or inverting input terminal of current feedback amplifier 600.
- the source of input transistor device 602 can be coupled to the output of the pass device in a feedback arrangement, and the source of input transistor device 604 can be coupled to the output of the error amplifier.
- Input device 602 has a gate coupled to a gate of a diode-connected transistor device 606, while input device 604 has a gate coupled to a gate of a diode-connected transistor device 608.
- Resistor R D1 is coupled between pass device 704 and resistor R D2 , while resistor R D2 is connected to ground.
- a composite feedback signal can be provided from a node V FDBK configured between resistors R D1 and R D2 , to the negative input terminal of error amplifier 702, i.e., to the gate of input transistor 710.
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Description
- The present invention relates to power supply circuits. More particularly, the present invention relates to a low drop-out regulator having composite amplifier configured to provide a higher performance power supply circuit.
- The increasing demand for higher performance power supply circuits has resulted in the continued development of voltage regulator devices. Many low voltage applications are now requiring the use of low dropout (LDO) regulators, such as for use in cellular phones, pagers, laptops, camera recorders and other mobile battery operated devices. These portable electronics applications typically require low voltage and quiescent current flow to facilitate increased battery efficiency and longevity. The alternative to low drop-out regulators are switching regulators which operate as dc-dc converters. Switching regulators, though similar in function, are not preferred to low drop-out regulators in many applications because switching regulators are inherently more complex and costly, i.e., switching regulators can have higher cost, as well as increased complexity and output noise than low drop-out regulators.
- Low drop-out regulators generally provide a well-specified and stable dc voltage whose input to output voltage difference is low. Low drop-out regulators typically have an error amplifier in series with a pass device, e.g., a power transistor, which is connected in series between the input and the output terminals of the low drop-out regulator. The error amplifier is configured to drive the pass device, which can then drive an output load. The operation of the low drop-out regulator is based on a control loop, which includes the feeding back of an amplified error signal used to control the output current flow of the power transistor driving the output load. The drop-out voltage of the low drop-out regulator is defined as the value of the input/output differential voltage that the control loop stops regulating. Low drop-out regulator also typically requires large output capacitors that are required to have a low electrical series resistance (ESR). However, such capacitors tend to require large circuit board area, and thus are highly responsible for the overall cost of the low drop-out regulator. Such a low drop-out regulator generally has two inherent characteristics including the magnitude of the input voltage being greater than the respective output voltage, and the output impedance being low so as to yield good performance. Low drop-out regulators can also typically be categorized as either low power or high power. Low power low drop-out regulators generally have a maximum output current of less than 1A, and are used mainly by the above portable applications. On the other hand, high power low drop-out regulators can yield currents that are equal to or greater than 1A at the output, which can be demanded by many automotive and industrial applications.
- With reference to
Figure 1 , a schematic diagram of a conventional low drop-out regulator 100 is illustrated. Low drop-outregulator 100 includes anerror amplifier 102 and apass device 104 configured in a feedback arrangement.Error amplifier 102 is configured to drive a low current during DC conditions, and a high current, e.g., 1 mA, under high slew or transient conditions. Error amplifier typically comprises a class AB-type amplifier device.Error amplifier 102 has a positive input connected to a reference voltage VREF, and powered by an input supply voltage VIN. Reference voltage VREF, which usually comprises a zener diode for high voltage applications or a bandgap reference for low voltage and high accuracy applications, is configured to provide a stable dc bias voltage with limited current driving capabilities. -
Pass device 104 comprises a power transistor device MP configured for driving an output current IOUT to a load device.Pass device 104 has a control terminal suitably coupled to the output oferror amplifier 102 and can comprise various configurations, such as NPN follower, NMOS follower, or common emitter PNP or common source PMOS transistors. Bipolar devices are generally used for applications requiring higher output currents and are capable of generating higher quiescent currents, while MOS devices are generally used for applications requiring minimized quiescent current. For bipolar devices, the beta β is defined as the ratio of the collector current to base current. This base current can be large and is often driven into ground, i.e., the ground current is increased considerably. For a low drop-out regulator, beta is also a measure of the efficiency, i.e., the ratio of the output current IOUT to the ground current. Because the bipolar device is considered a current gain device, the beta β can be quite low, ranging approximately from 100 to 1000. Thus, for every milliamp of current delivered at the output IOUT, 1µA to 10µA would be delivered to ground, i.e., for 100mA of output current, between 100µA and 1000µA of ground current are realized, resulting in poor efficiency for such bipolar devices. - Accordingly, CMOS transistor pass devices are usually the best overall configuration for optimizing efficiency. In the example of
Figure 1 ,pass device 104 comprises a PMOS transistor device, which typically requires very low DC current under full load conditions.Pass device 104 receives at a control terminal, e.g., gate terminal, an amplified error signal fromerror amplifier 102 configured to control the output current flow ofpass device 104 when driving the output load at an output terminal VOUT. Pass device 104 is configured to feed back the error signal toerror amplifier 102. -
Pass device 104 also introduces large, parasitic capacitances C1 and C2 to low drop-outregulator 100. The large capacitances, for example 100pF or more, can limit the capability oferror amplifier 102, since the capacitances require high current during a fast transition. For example, when designing devices configured to respond rapidly to changes in the output load,pass device 104 requires a large amount of current since parasitic capacitances C1 and C2 must be charged and discharged. Thus, in transient conditions, milliamps of current during microsecond periods must be supplied byerror amplifier 102 just to charge parasitic capacitances C1 and C2. - In addition to the requirement for higher current during transient conditions, other constraints are present on
error amplifier 102. For example, as currently available power systems are demanding the use of less operating supply voltage VIM, such as an operating voltage of 1.8 volts, low drop-out regulator has to operate within one gate-source voltage VGS, or approximately within a threshold voltage VT of the pass device plus an extra voltage Δ. Thus for a single gate-source voltage VGS topology, to turn onpass device 104 with a threshold voltage VT of 0.7 to 1.2 volts,error amplifier 102 must provide at least that voltage plus the extra voltage Δ, all within the limited headroom of 1.8 volts. - Another constraint on
error amplifier 102 is the need to control the offset of the low drop-out regulator. In other words, not only doeserror amplifier 102 need to comprise a class AB device that can drive a lot of output current, while also providing a low quiescent current during low voltages,error amplifier 102 also needs to minimize the offset contribution. - Yet another constraint of
error amplifier 102 is the compensation requirement. As discussed above,pass device 104 includes large parasitic capacitances, thus often requiring the implementation of a buffer, or a gm boost, to isolate the high output resistance of the gain stage oferror amplifier 102 from the high load capacitance ofpass device 104. For example, with reference toFigure 2 , a low drop-out regulator 200 implementing abuffer 206 between the output of anerror amplifier 202 and apass device 204 is illustrated.Buffer 206 is configured to receive the output current fromerror amplifier 202 and drive the gate ofpass device 204. The output frombuffer 206 can be mirrored back through a complex, current mirror circuit comprising transistors M1 through M5 to compensateerror amplifier 202. Other schemes can further incorporate an additional operational amplifier at the output of the pass device to sense the output current. However, adding such additional components can create stability problems. In addition, low drop-outregulator 200 generally has a lower efficiency due to a higher ground current, i.e., the current mirror circuit comprising transistors M1 through Ms tends to drive current to ground. - In some applications, with reference to a low drop-
out regulator 300 illustrated inFigure 3 , abuffer 306 can comprise a bipolar follower configuration, which is biased in class A operation. However, in either compensation scheme, current is being taken from the supply and driven into ground, i.e., the ground current is increased considerably, resulting in reduced efficiency. - In addition, for bipolar buffer configurations, headroom limitations are often readily apparent. For example, to
buffer error amplifier 302 and to drivepass device 304,NPN follower device 306 needs to be at least a base-emitter voltage VBE above the drive voltage, i.e., level shifting of the voltage at the gate ofpass device 304 is necessary. Thus, for a drive voltage of 0.8 volt needed to drive the gate ofpass device 304, and with a base-emitter voltage VBE of 0.8 to 1.0 volt, very little headroom is available for lower voltage power supply circuits, such as those with supply voltages VIN of 1.8 volts. As a result, control ofpass device 304 can be difficult. - Accordingly, a need exists for a low drop-out regulator that provides higher performance and efficiency, and that can overcome the various problems with prior art low drop-out regulators.
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US patent no. 5563501 describes a low voltage dropout regulation circuit where the internal compensating capacitance coupled to the regulated output port is coupled to a virtual ground and the virtual ground is current buffered for coupling to the control electrode of the path element. For additional frequency compensation, particularly when the path element has a large capacitance, an additional internal compensating capacitance is coupled between the input of the dropout circuit and an output of a transconductance amplifier, which is responsive to the voltage at the regulated output port. - The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with various aspects of the present invention, a low drop-out regulator is configured to provide high output current with a fast response during transient conditions, while also maintaining low quiescent current under DC conditions.
- The present invention accordingly provides a low drop-out regulator configured for providing output current to a load device, and a digital signal processor comprising the same, as set forth in the appended claims.
- A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and:
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Figure 1 illustrates a block diagram of a prior art low drop-out regulator; -
Figure 2 illustrates a block diagram of a prior art low drop-out regulator incorporating a buffer configuration; -
Figure 3 illustrates a block diagram of another prior art low drop-out regulator incorporating a NPN follower; -
Figure 4 illustrates a block diagram of a low drop-out regulator in accordance with an exemplary embodiment of the present invention; -
Figure 5 illustrates a schematic diagram of an exemplary embodiment of an error amplifier in accordance with the present invention; -
Figure 6 illustrates a schematic diagram of an exemplary embodiment of a current feedback amplifier in accordance with the present invention; and -
Figure 7 illustrates a schematic diagram of an exemplary embodiment of a low drop-out regulator in accordance with the present invention. - The present invention may be described herein in terms of various functional components and various processing steps. It should be appreciated that such functional components may be realized by any number of hardware or structural components configured to perform the specified functions. For example, the present invention may employ various integrated components, such as buffers, current mirrors, and logic devices comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes and the like, whose values may be suitably configured for various intended purposes. In addition, the present invention may be practiced in any integrated circuit application. However for purposes of illustration only, exemplary embodiments of the present invention will be described herein in connection with a low drop-out regulator for use with power supply circuits. Further, it should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection through other components and devices located thereinbetween.
- As discussed above, prior art low drop-out regulators have difficulty in maintaining low quiescent current flow, as well as in controlling offset contributions. Further, prior art low drop-out regulators utilize complex compensation schemes, and have difficulty controlling the pass device when limited headroom is available. However, in accordance with various aspects of the present invention, a low drop-out regulator is configured to provide high output current with a fast response during transient conditions, while also maintaining low quiescent current.
- In accordance with an exemplary embodiment, an exemplary low drop-out regulator comprises an error amplifier, a current feedback amplifier, and a pass device. The low drop-out regulator includes a composite amplifier feedback configuration, with the current feedback amplifier being decoupled from the overall composite feedback configuration. As a result, the current feedback amplifier can be configured to operate with low current supplied from the error amplifier and to drive the gate of the pass device with sufficiently high current as demanded by a load device.
- With reference to
Figure 4 , an exemplary low drop-out regulator 400 in accordance with an exemplary embodiment of the present invention is illustrated. Low drop-out regulator 400 suitably comprises anerror amplifier 402, apass device 404, acurrent feedback amplifier 406, and adivider network 408. Low drop-out regulator 400 includes a composite amplifier feedback configuration, with the feedback loop ofcurrent feedback amplifier 406 being decoupled from the overall composite feedback loop, and withcurrent feedback amplifier 406 being configured to provide effective compensation. - In accordance with the exemplary embodiment,
error amplifier 402 suitably comprises a class A device configured to control the gain and offset of low drop-out regulator 400.Error amplifier 402 includes a non-inverting input terminal configured to receive a reference voltage, such as a bandgap reference voltage VBG, and a negative input terminal configured to receive a composite feedback signal fromresistor network 408 through a resistor device RC. In addition, a capacitor CC is coupled from the output oferror amplifier 402 to the negative input terminal oferror amplifier 402. Capacitor CC can also be configured to supply additional current to the input ofcurrent feedback amplifier 406 to facilitate the driving of the control terminal ofpass device 404 during transient conditions, e.g., for driving of the control terminal ofpass device 404 to ground when the output voltage at output terminal VOUT is pulled down. As will be discussed further below,error amplifier 402 is not required to drive a large amount of current to operatecurrent feedback amplifier 406. -
Current feedback amplifier 406 is configured to operate with low current fromerror amplifier 402 and to suitably drive the control terminal ofpass device 404. In the exemplary embodiment,current feedback amplifier 406 is configured to receive an output signal fromerror amplifier 402 at an inverting input terminal. Current feedback amplifier also comprises a unity gain buffer configured withpass device 404 and a local feedback loop coupled from an output ofpass device 404 to a non-inverting input terminal, i.e., a current feedback loop decoupled from the composite amplifier loop. As discussed further below,current feedback amplifier 406 can be configured in various circuit arrangements for drivingpass device 404. -
Pass device 404 comprises a power transistor device configured for driving an output current IOUT to a load device. In the exemplary embodiment,pass device 404 comprises a PMOS transistor device having a source coupled to a supply voltage rail VS. a drain coupled to a output voltage terminal VOUT, and a control terminal, i.e., a gate, coupled to the output ofcurrent feedback amplifier 406. However, pass device can comprise any power transistor configuration, such as NPN or NMOS follower transistors, or any other transistor configuration for driving output current IOUT to a load device.Pass device 404 is configured to source as much current as needed by the load device and/ordivider network 408. -
Divider network 408 suitably comprises a resistive divider configured for providing a composite feedback signal. In the exemplary embodiment,divider network 408 comprises a pair of resistors RD1 and RD2. Resistor RD1 is coupled betweenpass device 404 and resistor RD2, while resistor RD2 is connected to ground. A composite feedback signal can be provided from a node VFDBK configured between resistors RD1 and RD2, through resistor RC to the inverting input terminal oferror amplifier 402. - During operation, under normal DC conditions where the output current IOUT at output terminal VOUT is in a steady state,
error amplifier 402 is configured to provide an output voltage equal to the voltage at output voltage terminal VOUT, and a low output current, to the inverting input terminal ofcurrent feedback amplifier 406. When a transient event occurs at the output load, e.g., an increase or decrease in output current IOUT demanded by the output load,current feedback amplifier 406 is configured to provide a high output current to drivepass device 404, while only receiving a low input current fromerror amplifier 402 and a high transient current provided by capacitor CC. - In accordance with another aspect of the present invention, the current error amplifier can be configured to permit the voltage at the control terminal of the pass device, e.g., the gate voltage of a PMOS transistor device MP, to operate from rail-to-rail. To understand the operation of the error amplifier, an illustration of a
basic error amplifier 500 for driving anoutput device 502 is illustrated inFigure 5 . Inerror amplifier 500, when utilizing a high impedance input configuration, a bandgap reference voltage VBG and feedback voltage VFDBK will appear to be present at the gate terminals of a pair ofPMOS transistor devices PMOS devices output device 502 include large, parasitic capacitances that need to be charged to facilitate driving of current through a currentmirror comprising transistor pass device 502. In addition, each ofPMOS devices pass device 502 are configured as large transistor devices, such as 10x devices. However, the large capacitances tend to limit the amount of current that can be driven on the output. Further, the amount of output current that can be driven is also limited by acurrent source 512 configured at the drain ofPMOS device 502. - However, in accordance with an exemplary embodiment, instead of providing the feedback and reference signals into the high impedance gates of a pair of input devices, a current feedback amplifier is configured with a feedback and/or reference signal being provided to the low impedance source terminals of a pair of input devices. As a result, current is forced through the pair of input devices and can be suitably utilized to supply the low drop-out regulator with the ability to provide fast, rail-to-rail output drive capabilities from an output device of the current feedback amplifier to the pass device.
- For example, with reference to
Figure 6 , an exemplarycurrent feedback amplifier 600 in accordance with an exemplary embodiment of the present invention is illustrated.Current feedback amplifier 600 is configured to provide a fast response while also being configured to drive large amounts of current to a pass device. In addition,current feedback amplifier 600 can facilitate rail-to-rail operation of the gate of the pass device.Current feedback amplifier 600 suitably comprises a pair ofinput transistor devices devices current mirrors current sources current mirror 620. -
Input transistor devices input transistor device 602 comprising the positive or non-inverting input terminal and the source ofinput transistor device 604 comprising the negative or inverting input terminal ofcurrent feedback amplifier 600. In accordance with the exemplary embodiment, the source ofinput transistor device 602 can be coupled to the output of the pass device in a feedback arrangement, and the source ofinput transistor device 604 can be coupled to the output of the error amplifier.Input device 602 has a gate coupled to a gate of a diode-connectedtransistor device 606, whileinput device 604 has a gate coupled to a gate of a diode-connectedtransistor device 608. In addition,input device 602 has a drain coupled tocurrent mirror 610, whileinput device 604 has a drain coupled tocurrent mirror 616. Diode-connecteddevices input devices devices input devices input devices device 606 has source coupled to input voltage signal VNN(-) similar to the connection of the source ofinput device 604, and a drain coupled to ground through acurrent source 626, while diode-connecteddevice 608 has a source coupled to input voltage signal VPP(+) similar to the connection of the source ofinput device 602, and a drain coupled to ground through acurrent source 628. While diode-connecteddevices input devices input devices devices -
Current sources devices Current sources devices devices input devices devices input devices -
Current mirrors transistors current mirror 620, a diode-connectedtransistor 622 and anupper output device 624 configured at the upper rail ofcurrent feedback amplifier 600.Current mirror 610 comprises a diode-connectedtransistor 614 having a drain coupled toinput device 602, and atransistor 612 having a gate coupled to a gate oftransistor 614, and a drain coupled toupper transistor 622. Likewise,current mirror 616 comprises a diode-connectedtransistor 618 having a drain coupled toinput device 604, and alower output device 620 having a gate coupled to a gate oftransistor 618, and a drain coupled toupper output device 624. -
Upper rail transistors transistor 622 is configured to suitably mirror any current received fromcurrent mirror 610 to the control terminal of the pass device through the drain oftransistor 624, which comprises the upper output device forcurrent feedback amplifier 600. In accordance with an exemplary embodiment,upper output device 624 is approximately four times the transistor size ofupper rail transistor 622, e.g., an 8X device size foroutput device 624 and a 2X device size fortransistor 622.Lower output device 620 is also sized approximately four times the transistor size oflower rail transistor 618. - Accordingly,
current feedback amplifier 600 is configured to operate with a very low queue current ininput devices error amplifier 500,current feedback amplifier 600 can supply additional current through theoutput devices - Having described the configuration and operation of an exemplary current feedback amplifier, an implementation of an exemplary current feedback amplifier within a low drop-out regulator can be provided. With reference to
Figure 7 , an exemplary low drop-out regulator 700 is illustrated in accordance with an exemplary embodiment of the present invention. Low drop-out regulator 700 comprises anerror amplifier 702, apass device 704, acurrent feedback amplifier 706, and a divider network 708. Low drop-out regulator 700 is suitably configured with a composite feedback loop, with the feedback loop ofcurrent feedback amplifier 706 being decoupled from the overall composite feedback loop. - In accordance with this exemplary embodiment,
error amplifier 702 suitably comprises a class A device configured to control the gain and offset of low drop-out regulator 700.Error amplifier 702 includes a differential pair oftransistors 710 and 712, acurrent source circuit 726, and anoutput device 724. Transistor 712 has a gate configured as a positive input terminal coupled to a reference voltage VREF, such as a bandgap reference voltage, e.g., a bandgap voltage of approximately 1.2 volts.Transistor 710 has a gate configured as a negative input terminal configured to receive a composite feedback signal from resistor network 708. Source terminals oftransistors 710 and 712 are connected together within the differential pair configuration, and are coupled to acurrent source 716. Drains of differential pair oftransistors 710 and 712 can be coupled to a gate ofoutput transistor 724 through a currentmirror comprising transistors input transistor 710 can be suitably mirrored through diode-connectedtransistor 720 andtransistor 722 to the gate ofoutput transistor 724, while output current from input transistor 712 can be directly connected to drive the gate ofoutput transistor 724. - A composite amplifier feedback loop can be provided from divider network 708 through a resistor device RC to the negative terminal of
error amplifier 702, i.e., to the gate ofinput transistor 710. Resistor device RC can comprise various resistance values to effectively vary the compensation toerror amplifier 702. In addition,error amplifier 702 can comprise a capacitor CC coupled between the negative terminal oferror amplifier 702, i.e., to the gate ofinput transistor 710, and the drain ofoutput device 724 to suitably supply additional current to facilitate the driving of the gate ofpass device 704 during transient conditions, e.g., for driving of the gate ofpass device 704 to ground when the output voltage at output terminal VOUT is pulled down. Capacitor CC can vary in capacitance level between approximately 1Op to 100pF, with the higher the value of capacitance, the lower the value of resistance of resistor RE. In accordance with an exemplary embodiment, resistor device RC can comprise an active variance device, while capacitor CC comprises a fixed capacitance of approximately 20pF. - A supply voltage VS is suitably configured to supply voltage to the upper supply rail. Supply voltage VS can comprise various levels of supply voltage, such as 2.8 volts, that provides additional headroom. However, supply voltage VS can also comprise a significantly lower voltage supply, such as 1.8 volts, and yet have sufficient headroom for operation of low drop-
out regulator 700. -
Current source device 726 is configured to drive a plurality ofcurrent sources Current source device 726 can comprise various types and configurations of current source devices for driving a plurality of current sources. To facilitate the driving ofcurrent sources error amplifier 702 can include a diode-connectedtransistor device 714 configured to mirror current fromcurrent source device 726 tocurrent sources current feedback amplifier 600, due to the operation ofcurrent sources error amplifier 702 is not required to drive a large amount of current to operatecurrent feedback amplifier 706. -
Current feedback amplifier 706 is configured to operate with low current fromerror amplifier 702 and to suitably drive the gate ofpass device 704. In the exemplary embodiment,current feedback amplifier 406 comprises an amplifier similar to that ofcurrent feedback amplifier 600. However,current feedback amplifier 706 can also be configured in various other circuit arrangements configured for drivingpass device 704. In this exemplary embodiment,input device 602 has a source coupled through input terminal VPP(+) to output terminal VOUT and to passdevice 704, whileinput device 604 has a source coupled through input terminal VNN(-) tooutput device 724 oferror amplifier 702.
Pass device 704 comprises a power transistor device configured for driving an output current IOUT to a load device. In the exemplary embodiment,pass device 704 comprises a PMOS transistor device having a source coupled to a supply voltage rail VS, and a drain coupled to a output voltage terminal VOUT. However, pass device can comprise any power transistor configuration for driving output current IOUT to a load device. In addition,pass device 704 is configured to source as much current as needed by the load device and/or divider network 708. Divider network 708 suitably comprises a resistive divider configured for providing a composite feedback signal. In the exemplary embodiment, divider network 708 comprises a pair of resistors RD1 and RD2. However, divider network 708 can comprise any configuration of resistors for providing a voltage divider operation. Resistor RD1 is coupled betweenpass device 704 and resistor RD2, while resistor RD2 is connected to ground. A composite feedback signal can be provided from a node VFDBK configured between resistors RD1 and RD2, to the negative input terminal oferror amplifier 702, i.e., to the gate ofinput transistor 710. - In the exemplary embodiment, the positive input terminal of
current feedback amplifier 706, i.e., the source ofinput transistor 602, is coupled in a feedback arrangement to the output terminal VOUT and to the drain ofpass device 704. In addition, diode-connecteddevices input devices input devices - During operation of low drop-
out regulator 700, for example when the output voltage at terminal VOUT increases rapidly, such as when an output load is turned off rapidly to release the output voltage upwards,current amplifier 706 operates to drive node VGATE ofpass device 704 to the upper rail supply VS. - Since the load current is significantly reduced,
pass device 704 will suitably drive a higher current intoinput device 602 through input terminal VPP(+). The higher current flowing throughinput device 602 can be suitably mirrored throughcurrent mirror 610 toupper rail device 622, turning onoutput device 624, pulling up node VGATE very rapidly towards supply rail VS. -
Node 632 tracks the rise in VPP(+), since the gate-source voltage VGS ofdevice 608 remains fixed. The gate-source voltage VGS ofdevice 604 decreases, effectively shutting offlower output device 620, releasing node VGATE to rise even closer to the upper supply rail VS.
At the same time, the rising voltage at terminal VOUT is divided down by RD1 and RD2. The VFDBK node also rises, causing theoutput device 724 oferror amplifier 702 to shut off rapidly, thus causing the voltage at input terminal VNN(-) to decrease. The current driven throughinput device 604 is further reduced, releasing node VGATE to rise even closer to the upper supply rail VS. -
Node 630 tracks the decrease in VNN(-), since the gate-source voltage VGS ofdevice 606 remains fixed. The gate-source voltage VGS ofdevice 602 increases, further increasing the current throughdevice 602, which is suitably mirrored throughcurrent mirror 610 to theupper rail device 622, further turning onoutput device 624 to pull up the gate ofpass device 704. - On the other hand, when the output voltage at terminal VOUT decreases rapidly, such as when an output load is turned on rapidly to pull the output voltage downwards,
current amplifier 706 operates to drive node VGATE ofpass device 704 to ground. - Since the load current is significantly increased,
pass device 704 will suitably drive a lower current intoinput device 602 through input terminal VPP(+). The lower current flowing throughinput device 602 can be suitably mirrored throughcurrent mirror 610 toupper rail device 622, effectively turning offoutput device 624, releasing node VGATE to fall to ground. -
Node 632 tracks the fall in VPP(+), since the gate-source voltage Vgs ofdevice 608 remains fixed. The gate-source voltage VGS ofdevice 604 increases, turning onlower output device 620, pulling down node VGATE very rapidly towards ground. - At the same time, the decreasing voltage at terminal VOUT is divided down by RD1 and RD2. The VFDBK node also decreases, causing the
output device 724 oferror amplifier 702 to turn on rapidly, causing the voltage at input terminal VNN(-) to increase. The current driven throughinput device 604 is further increased, pulling down node VGATE even closer to ground. -
Node 630 tracks the increase in VNN(-), since the gate-source voltage VGS ofdevice 606 remains fixed. The gate-source voltage VGS ofdevice 602 decreases, further decreasing the current throughdevice 602, which is suitably mirrored throughcurrent mirror 610 to theupper rail device 622, further turning offoutput device 624, thus releasing node VGATE to fall to even closer to ground. - As a result, the high current provided to the gate of
pass device 704 suitably enables any parasitic capacitances withinpass device 704 to be rapidly charged and discharged without impairing the operation oferror amplifier 702 andcurrent feedback amplifier 706. In addition, the gate ofpass device 704 can be suitably driven to the upper rail and ground, i.e., rail-to-rail, with the current supplied bycurrent feedback amplifier 706. Accordingly,current feedback amplifier 706 can suitably utilize current, rather than voltage, to charge and discharge the parasitic capacitances very rapidly. In other words,current feedback amplifier 706 can suitably receive an input current, convert that current into a voltage, and then convert the voltage back to a current for output to drive the pass device. - Moreover,
current feedback amplifier 706 does not require a high input voltage or high input current for operation. Instead, a low voltage less than 2 times the threshold voltage VT, can be provided toerror amplifier 702 andcurrent feedback amplifier 706. In addition,current feedback amplifier 706 can operate with only a few micro-amps of current, and yet can provide several milli-amps of output current very quickly to drive the gate ofpass device 704. - In accordance with another aspect of the present invention, the gain of low drop-
out regulator 700 can be relegated toerror amplifier 702, which also controls the offset, and which does not need to drive a high amount of current tocurrent feedback amplifier 706. In accordance with this aspect of the present invention, the matching of the various transistor devices ofcurrent feedback amplifier 706, such asdevices error amplifier 702, such asdevices 710 and 712, is not critical to the operation of low drop-out regulator 700. The composite feedback configuration oferror amplifier 702, which is configured to control the offset of low drop-out regulator 700, does not significantly affect the accuracy of the output ofcurrent feedback amplifier 706. In addition, the gain of low drop-out regulator 700 is controlled byerror amplifier 702, i.e.,current feedback amplifier 706 does not need to control the gain, and thus compensation does not need to be provided fromcurrent feedback amplifier 706. Accordingly,transistor devices 710 and 712 can comprise 10x devices, whiledevices 602 and 604 (4x),devices 606 and 608 (2x), anddevices 612 and 614 (1x) can comprise different sized devices without impacting the offset of low drop-out regulator 700. - The present invention has been described above with reference to various exemplary embodiments. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiments without departing from the present invention as claimed. For example, the various components may be implemented in alternate ways, such as, for example, by implementing BJT devices for the various devices. Further, the various exemplary embodiments can be implemented with other types of power supply circuits in addition to the circuits illustrated above. These alternatives can be suitably selected depending upon the particular application or in consideration of any number of factors associated with the operation of the system.
Claims (9)
- A low drop-out regulator (400) configured for providing output current to a load downstream device, said low drop-out regulator comprising:an error amplifier (402) configured to control the gain of said low drop-out regulator, said error amplifier having a positive input terminal configured for receiving a reference voltage (VBG), and a negative input terminal configured for receiving a composite feedback signal from a first feedback loop;a pass device (404) configured for driving the downstream device, said pass device comprising a power transistor configured to drive a load current to the downstream device, said pass device having a source terminal coupled to a voltage source (VSS); anda divider network (408) configured for receiving an output signal of said pass device (404), andcharacterized by:a current feedback amplifier (406) having a negative input terminal coupled to an output of said error amplifier (402), and a positive input terminal configured for receiving a feedback signal from a local feedback loop;the pass device (404) having a control terminal coupled to an output of said current feedback amplifier (406), the local feedback loop coupling the output of the pass device (404) to the positive input terminal of the current feedback amplifier (406);a capacitor (CC) coupled between said output of said error amplifier (402) and said negative input terminal of said error amplifier (406), said capacitor (CC) configured for supplying current to said current feedback amplifier (406) for driving said pass device (404); andthe divider network (408) configured for generating said composite feedback signal and providing said composite feedback signal along a path through a resistance device to said negative input terminal of said error amplifier (402), the first feedback loop comprising said path.
- The low drop-out regulator (400) according to claim 1,
wherein said error amplifier (402) comprises a class A output configuration for providing a low current from said error amplifier to said current feedback amplifier (406). - The low drop-out regulator (400) according to claim 1 or 2, wherein said error amplifier (500) comprises: ,a differential pair of transistors (506,508);a current mirror circuit (501, 503) configured for mirroring current from a drain of one of said differential pair of transistors (506, 508); andan output transistor (502) having a control terminal configured for receiving said current mirrored from said current mirror circuit and current from an output of another of said differential pair of transistors (506, 508).
- The low drop-out regulator (400, 700)) according to any one of claims 1 to 3, wherein said current feedback amplifier (600, 706)) is configured for rail-to-rail operation of said control terminal of said pass device (404, 704), said current feedback amplifier (600, 706)) comprising:a pair of input transistors (602, 604), with a first input transistor (602) having an input terminal configured for receiving said feedback signal within said local feedback loop, and a second input transistor (604) having an input terminal configured for receiving a low current from said error amplifier (402, 702);a pair of diode-connected devices (606, 608) configured to control a flow of current within said pair of input transistors (602, 604), with a first diode-connected device (606) having a control terminal connected to a control terminal of said first input transistor (602), and a second diode-connected device (608) having a control terminal connected to a control terminal of said second input transistor 604);a pair of current mirrors (610, 616) configured to mirror said flow of current with said pair of input transistors (602, 604), with a first current mirror (610) coupled to an output terminal of said first input transistor (602), and a second current mirror (616) coupled to an output terminal of said second input transistor (604); anda pair of upper rail transistors (622, 624) configured for providing said high output current to said pass device (404, 704), said pair of upper rail transistors comprising a first upper rail transistor (622) configured for mirroring current from said first current mirror (610) to a second upper rail transistor (624) comprising an output device for said current feedback amplifier (600) configured for providing said high output current.
- The low drop-out regulator (400, 700) according to claim 6, wherein said current feedback amplifier (600) further comprises:a first current source (626) coupled to an output terminal of said first diode-connected device (606) and being configured to provide a fixed current flowing through said first diode connected device; anda second current source (628) coupled to an output terminal of said second diode-connected device (608), said second current source being configured to provide a fixed current flowing through said second diode connected device.
- The low drop-out regulator (400, 700) according to claim 4 or 5, wherein said current feedback amplifier (600) is configured to provide rail-to-rail operation by turning on said first input transistor (602) and turning off said second input transistor (604) to drive said control terminal of said pass device (404, 704)to an upper rail, and by turning off said first input transistor (602) and turning on said second input transistor (604) to drive said control terminal of said pass device to ground.
- The low drop-out regulator (400, 700) according to any one of claims 4 to 6, wherein said pair of input transistors (602, 604) is approximately twice the size of said pair of diode-connected devices (606, 608).
- The low drop-out regulator (400, 700) according to any one of claims 4 to 7, wherein said output device (624) of said current feedback amplifier (600, 706) is approximately four times the size of said first upper rail transistor (622).
- A digital signal processor having a low drop-out regulator configured for providing output current to a load device according to any one of claims 1 to 8.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US10/151,366 US6703815B2 (en) | 2002-05-20 | 2002-05-20 | Low drop-out regulator having current feedback amplifier and composite feedback loop |
US151366 | 2002-05-20 |
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EP1365302A1 EP1365302A1 (en) | 2003-11-26 |
EP1365302B1 true EP1365302B1 (en) | 2010-04-28 |
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EP03101432A Expired - Lifetime EP1365302B1 (en) | 2002-05-20 | 2003-05-20 | Low drop-out voltage regulator |
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US (1) | US6703815B2 (en) |
EP (1) | EP1365302B1 (en) |
JP (1) | JP2004005670A (en) |
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Also Published As
Publication number | Publication date |
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DE60332300D1 (en) | 2010-06-10 |
US20030214275A1 (en) | 2003-11-20 |
JP2004005670A (en) | 2004-01-08 |
US6703815B2 (en) | 2004-03-09 |
EP1365302A1 (en) | 2003-11-26 |
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